It has been shown that surface relief structures of diffractive elements (or indeed various different surface relief patterns or functions) can be formed into suitable receptive material by any one of many known methods.
FIG. 1 of the drawings shows the basic structure of an embossed holographic or diffractive security device, referred to above as a type ‘A’ device. As shown, the device comprises a substrate layer 10 of a UV curable resin or an embossable thermoplastic. The substrate carries a micro-relief grating pattern 12 and a very thin layer of metal or multi-layer reflective coating 14 (eg vacuum deposited). An upper layer 16 of UV curable resin or other isotropic material with a refractive index n of typically around 1.45 to 1.6 is coated onto the substrate. In such an arrangement, with an embossed surface with pitch p, the direction of the diffracted orders is determined by:—Sin(θm)=Sin(θo)+m·λ/p  (Equation (1))
Where λ is the wavelength of the diffracted light, p is the grating pitch (period) and m is the diffraction order. θm and θo correspond to the angles between the normal to the reflection surface and the directions of orders m and o. when m=0, (zero order), this corresponds to the mirror reflection (undiffracted light).
In practical devices used in security applications (holograms, diffractive gratings) typical parameters are:
D (structure depth) about 0.1 to 0.5 μm
p (structure pitch/period) about 0.4 to 2.0 μm
In FIG. (1) for the first order sin (α)=λ/p, where α is the angle between the light diffracted into the first order and the mirror reflection (zero order). For λ=0.5 μm (average of visible light), and p=1 μm, the above expression gives α=30°. It should be noted that for the special case of vertical illumination the values θo=0, θm=α are obtained. The diffractive effects are mainly due to the local orientations and spacings of the gratings. However, the profile-depths of the grating, to a first order, determine the diffraction efficiency.
Referring to FIG. 2, in another type of reflective mode device, instead of providing a reflective layer 14 between the substrate 10 and the upper layer 16, the surface of the substrate layer 10 from the interface may be made reflective, as shown by reflective layer 18 in FIG. 2. Similarly the upper surface of layer 16 may be rendered reflective. To have sufficient diffractive efficiency the refractive indices of the substrate layer 10 and the layer 16 must differ in order to avoid index matching.
The devices can be made to work in transmission. Referring to FIG. 3, in transmissive mode, both the upper layer 16 and the substrate 10 are transparent. To have sufficient diffractive efficiency, the refractive indices of substrate 10 (n1) and layer 16 (n2) must differ in order to avoid index matching. Practical curable resins can give:Δn=n1−n2=0.2n1˜1.4n2˜1.6
Therefore the depth of the structures should in general be larger than in the reflective case to get sufficient difference in the optical path. The other parameters and structures are similar to the reflective case.
The operation of type B devices which use liquid crystalline materials to provide optical phase modulation/retardation are described in e.g. “Optical LPP/LCP Devices—A New Generation of Optical Security Elements” Proceedings of SPIE Vol 3793 (2000), pp 196-203 and “New Coloured Optical Security Elements Using LPP/LCP Technology” Proceedings of SPIE Vol. 4672 (2002). The operation of a transmissive and reflective device is also explained in these references.